The reciprocal of a number is the number that the first number will multiply with that equals one.
So, for instance, the reciprocal of 3 is 1/3.
Since we can write 3 as 3/1, we can see it's simply flipping the numerator and denominator to find the reciprocal.
So, flipping numerator and denominator, the reciprocal of 31/2 is 2/31.